SLOTHS AND THEIR HAIR 319 
Of all the problems with which the naturalist has to 
deal, those connected with the “mimicry” of one animal 
by another, or the special resemblances by certain animals to 
their inanimate surroundings, are some of the most difficult, 
and the present instance forms no exception to this rule, if 
it is believed that “natural selection,” or some such mode 
of evolution, has been the sole factor in the case, 
In this instance, at any rate, there can be no question as 
to any volition on the part of the animal concerned having 
aided in the development of its protective resemblance. 
And, on the hypothesis of natural selection, it appears 
necessary to assume that when the modern type of sloths 
was first evolved no alga grew in the hair of these animals, 
which were consequently able to exist and flourish without 
any such adventitious aid. The nature of their hair formed, 
however, in the case of each of the two groups, a con- 
venient xidus for the lodgment and growth of an alga; 
and such a suitable situation was accordingly in each 
instance seized on as a habitat by one of those lowly 
plants. At first, of course, only a certain number of 
sloths would have had alga-producing hair, and these, 
from the green tinge of their coats, would consequently 
enjoy a better chance of escape from foes than would their 
brethren which had not yet acquired the greenish garb. 
And, on the assumption that alga-growing hair is in- 
herited, their progeny would consequently have the best 
chance of winning in life’s race. It is, of course, not 
difficult to assume that when the alga had once become 
firmly established as part and parcel of the hair of each 
group it acquired in both cases distinct specific characters, 
even if there were not originally two kinds of these plants 
concerned. 
And here arises one of the many difficulties connected 
ae ce te ae 
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